The present invention relates to the fabrication of semiconductor metamaterials.
Metamaterials are periodically repeating, synthetic composite structures that are specifically engineered to circumvent inconvenient bulk material properties. The exceptional characteristics and response functions of metamaterials are not observed in the individual constituent materials of the composite, and these phenomena arise as a direct result of the periodic inclusion of functional materials such as metals, semiconductors or polymers embedded within the composite. However, the fabrication of such structures is a serious experimental challenge as this full three-dimensional deposition and patterning requirement is extremely difficult to satisfy using conventional techniques such as chemical vapour deposition and photolithography.
Holey optical fibres exploit the concept of using a periodic array of air holes to define the transverse refractive index profile of the fibre. These fibres have exhibited exceptional optical properties that significantly outperform conventional fibre structures in key areas, and can guide light either by a modified form of total internal reflection or by exploiting photonic bandgap effects. This has generated enormous interest both within the academic and industrial communities due to novel optical properties that include endlessly single-mode guidance, anomalous dispersion, and mode area tailoring over three orders of magnitude that have many potential applications.
The inclusion of semiconductor materials into holey fibres and other engineered microstructured material to provide specifically tailored metamaterials is of significant technological interest as this allows easy integration into existing optoelectronic systems and devices.
Various techniques addressing this objective have been reported, such as the deposition of semiconductor materials onto fibre performs which are then pulled into optical fibres [1]. Direct bandgap bulk CdTe or CdS semiconductors were vacuum deposited onto a Pyrex (RTM) rod which was then inserted into a Pyrex (RTM) tube, thus creating a fibre perform, from which fibres were pulled, for use as light amplifiers for fibre optic communications or as non-linear devices. However, this technique suffers from many disadvantages. Vacuum deposition of semiconductor material onto a low melting temperature glass rod such as Pyrex (RTM) will result in a low quality amorphous layer, which when pulled to fibre will further degrade by reacting with the molten glass and forming a discontinuous film of material at the core-cladding boundary. Pure silica cannot be used in place of Pyrex (RTM) as the melting temperature would quickly vaporize most semiconductors during the pulling process.
Other examples of the incorporation of semiconductor materials into glass includes reports of quasicontinuous spectrally broad optical gain at cryogenic temperatures in CdS quantum dots embedded in borosilicate glass by melting [2], and in PbS quantum dot doped glasses fabricated by thermal treatment of an oxide molten glass which precipitated out the microcrystalline phase [3]. However, these techniques are also limited to use with low melting temperature glasses and suffer from chemical reactivity issues that reduce radiative efficiencies due to surface recombination, the presence of trap states, defects, dangling bonds, photodarkening and low quantum dot densities in the gain medium.
The fabrication of silicon nanocrystal quantum dots in a planar silica substrate has also demonstrated optical gain characteristics, with the use of quantum confined indirect bandgap silicon resulting in a broad ASE spectrum centered around 750 μM [4]. The devices were produced by ion implantation of silicon ions into ultrapure quartz wafers or thermally grown silicon dioxide layers on silicon substrates followed by a high temperature anneal. However, there was low photoluminescence efficiency and incomplete control over the size selection and distribution of the quantum dots, which, together with the planar geometry, mean that these devices may have limited practical use.
Fabrication of infrared waveguides by the impregnation of molten semiconductors into a silica capillary has been proposed, to exploit the high refractive index and ultra wide transmission window of the semiconductor [5]. However, the impregnation method implies using either capillary action or a vacuum process. This has several shortcomings, not least of which is the length over which material can be infused into the capillary, as this process is strongly determined by the properties of viscosity as a function of temperature, surface tension, glass wall adhesion characteristics and thermal expansion coefficients. Also, the choice of semiconductor is severely limited due to the typically high melting points of these materials.
Another example includes the development of germanium-coated nickel hollow waveguides for infrared transmission [6]. The fabrication technique involves placing an aluminum pipe inside a sputtering chamber, onto which is deposited a germanium layer followed by a metallic film. A thick nickel layer is then deposited on top of the sputtered layers in an electroplating tank. The pipe is then etched away leaving a hollow waveguide structure that can exploit Fresnel reflection to guide infrared radiation. A further example of hollow infrared waveguide structures is a process using a polymer-coated silica tubing, inside of which wet chemistry techniques including standard silver plating and iodisation were used to deposit metal and dielectric layers [7]. These two manufacturing methods are quite limited, however, since they result in high-loss waveguides only a few meters in length. The waveguides also suffer from poor reproducibility, additional loss on bending and have very poor mechanical properties, especially compared with conventional telecommunication fibres.
Thus, these various techniques for incorporating semiconductor material into microstructured fibres and other waveguides have many drawbacks, including the inability to fabricate long lengths of material, poor quality semiconductor deposition, and applicability to only a limited range of materials. Given the importance of photonic materials, in particular holey fibres, and their many potential applications, there is, therefore, a need for an improved fabrication technique.